Recently, display devices for performing image display are being developed. Liquid crystal display devices that perform image display by using a liquid crystal element are widely used as display devices because of advantages of high image quality, thinness, lightweight, and the like.
In addition, light emitting devices using self-light emitting elements as light emitting elements are recently being developed. The light emitting device has characteristics of, for example, a high response speed suitable for motion image display, low voltage, and low power consumption, in addition to advantages of existing liquid crystal display devices, and thus, attracts a great deal of attention as the next generation display device.
As gradation representation methods used in displaying a multi-gradation image on a light emitting device, an analog gradation method and a digital gradation method are given. The former analog gradation method is a method in which the gradation is obtained by analogously controlling the magnitude of a current that flows to a light emitting element. The latter digital gradation method is a method in which the light emitting element is driven only in two states thereof: an ON state (state where the luminance is substantially 100%) and an OFF state (state where the luminance is substantially 0%). In the digital gradation method, since only two gradations can be displayed, a method configured by combining the digital gradation method and a different method to display multi-gradation images has been proposed.
When classification is made based on the type of a signal that is input to pixels, a voltage input method and a current input method are given as pixel-driving methods. The former voltage input method is a method in which: a video signal (voltage) that is input to a pixel is input to a gate electrode of a driving element; and the driving element is used to control the luminance of a light emitting element. The latter current input method is a method in which the set signal current is flown to a light emitting element to control the luminance of the light emitting element.
Hereinafter, referring to FIG. 16(A), a brief description will be made of an example of a circuit of a pixel in a light emitting device employing the voltage input method and a driving method thereof. The pixel shown in FIG. 16(A) includes a signal line 501, a scanning line 502, a switching TFT 503, a driving TFT 504, a capacitor element 505, a light emitting element 506, and power sources 507 and 508.
When the potential of the scanning line 502 varies, and the switching TFT 503 is turned ON, a video signal that has been input to the signal line 501 is input to a gate electrode of the driving TFT 504. According to the potential of the input video signal, a gate-source voltage of the driving TFT 504 is determined, and a current flowing between the source and the drain of the driving TFT 504 is determined. This current is supplied to the light emitting element 506, and the light emitting element 506 emits light. As a semiconductor device for driving the light emitting element, a polysilicon transistor is used. However, the polysilicon transistor is prone to variation in electrical characteristics, such as a threshold value and an ON current, due to defects in a grain boundary. In the pixel shown in FIG. 16(A), if characteristics of the driving TFT 504 vary in units of the pixel, even when identical video signals have been input, the magnitudes of the corresponding drain currents of the driving TFTs 504 are different. Thus, the luminance of the light emitting element 506 varies.
To solve the problems described above, a desired current may be input to the light emitting element, regardless of the characteristics of the TFTs for driving the light emitting element. From this viewpoint, the current input method has been proposed which can control the magnitude of a current that is supplied to a light emitting element regardless of the TFT characteristics.
Next, referring to FIGS. 16(B) and 17, a brief description will be made of a circuit of a pixel in a light emitting device employing the current input method and a driving method thereof. The pixel shown in FIG. 16(B) includes a signal line 601, first to third scanning lines 602 to 604, a current line 605, TFTs 606 to 609, a capacitor element 610, and a light emitting element 611. A current source circuit 612 is disposed to each signal line (each column).
Operations of from video signal-writing to light emission will be described by using FIG. 17. In FIG. 17, reference numerals denoting respective portions conform to those shown in FIG. 16. FIGS. 17(A) to 17(C) schematically show current paths. FIG. 17(D) shows the relationship between currents flowing through respective paths during a write of a video signal, and FIG. 17(E) shows a voltage accumulated in the capacitor element 610 also during the write of a video signal, that is, a gate-source voltage of the TFT 608.
First, a pulse is input to the first and second scanning lines 602 and 603 to turn the TFTs 606 and 607 ON. A signal current flowing through the signal line 601 at this time will be referred to as Idata. As shown in FIG. 17(A), since the signal current Idata is flowing through the signal line 601, the current separately flows through current paths I1 and I2 in the pixel. FIG. 17(D) shows the relationship between the currents. Needless to say, the relationship is expressed as Idata=I1+I2.
The moment the TFT 606 is turned ON, no charge is yet accumulated in the capacitor element 610, and thus, the TFT 608 is OFF. Accordingly, I2=0 and Idata=I1 are established. In the moment, the current flows between electrodes of the capacitor element 610, and charge accumulation is performed in the capacitor element 610.
Charge is gradually accumulated in the capacitor element 610, and a potential difference begins to develop between both the electrodes (FIG. 17(E)). When the potential difference of both the electrodes has reached Vth (point A in FIG. 17(E)), the TFT 608 is turned ON, and I2 occurs. As described above, since Idata=I1+I2 is established, while I1 gradually decreases, the current keeps flowing, and charge accumulation is continuously performed in the capacitor element 610.
In the capacitor element 610, charge accumulation continues until the potential difference between both the electrodes, that is, the gate-source voltage of the TFT 608 reaches a desired voltage. That is, charge accumulation continues until the voltage reaches a level at which the TFT 608 can allow the current Idata to flow. When charge accumulation terminates (B point in FIG. 17(E)), the current I1 stops flowing. Further, since the TFT 608 is fully ON, Idata=I2 is established (FIG. 17(B)). According to the operations described above, the operation of writing the signal to the pixel is completed. Finally, selection of the first and second scanning lines 602 and 603 is completed, and the TFTs 606 and 607 are turned OFF.
Subsequently, a pulse is input to the third scanning line 604, and the TFT 609 is turned ON. Since VGS that has been just written is held in the capacitor element 610, the TFT 608 is already turned ON, and a current identical to Idata flows thereto from the current line 605. Thus, the light emitting element 611 emits light. At this time, when the TFT 608 is set to operate in a saturation region, even if the source-drain voltage of the TFT 608 varies, a light emitting current IEL flowing to the light emitting element 611 flows continuously.
As described above, the current input method refers to a method in which the drain current of the TFT 609 is set to have the same current value as that of the signal current Idata set in the current source circuit 612, and the light emitting element 611 emits light with the luminance corresponding to the drain current. By using the thus structured pixel, influence of variation in characteristics of the TFTs constituting the pixel is suppressed, and a desired current can be supplied to the light emitting element.
Incidentally, in the light emitting device employing the current input method, a signal current corresponding to a video signal needs to be precisely input to a pixel. However, when a signal line driver circuit (corresponding to the current source circuit 612 in FIG. 16) used to input the signal current to the pixel is constituted by polysilicon transistors, variation in characteristics thereof occurs, thereby also causing variation in characteristics of the signal current.
That is, in the light emitting element employing the current input method, variation in characteristics of TFTs constituting the pixel and the signal line driver circuit need to be suppressed. However, while the influence of variation in characteristics of the TFTs constituting the pixel can be suppressed by using the pixel having the structure of FIG. 16(B), suppression of the influence of variation in characteristics of the TFTs constituting the signal line driver circuit is difficult.
Hereinafter, using FIG. 18, a brief description will be made of the structure and operation of a current source circuit disposed in the signal line driver circuit that drives the pixel employing the current input method.
The current source circuit 612 shown in FIGS. 18(A) and 18(B) corresponds to the current source circuit 612 of FIG. 16(B). The current source circuit 612 includes constant current sources 555 to 558. The constant current sources 555 to 558 are controlled by signals that are input via respective terminals 551 to 554. The magnitudes of currents supplied from the constant current sources 555 to 558 are different from one another, and the ratio thereof is set to 1:2:4:8.
FIG. 18(B) shows a circuit structure of the current source circuit 612, in which the constant current sources 555 to 558 shown therein correspond to transistors. The ratio of ON currents of the transistors 555 to 558 is set to 1:2:4:8 according to the ratio (1:2:4:8) of the value of L (gate length)/W (gate width). The current source circuit 612 then can control the current magnitudes at 24=16 levels. Specifically, currents having 16-gradation analog values can be output for 4-bit digital video signals. Note that the current source circuit 612 is constituted by polysilicon transistors, and is integrally formed with the pixel portion on the same substrate.
As described above, conventionally, a signal line driver circuit incorporated with a current source circuit has been proposed (for example, refer to Non-patent Documents 1 and 2).
In addition, digital gradation methods include a method in which a digital gradation method is combined with an area gradation method to represent multi-gradation images (hereinafter, referred to as area gradation method), and a method in which a digital gradation method is combined with a time gradation method to represent multi-gradation images (hereinafter, referred to as time gradation method). The area gradation method is a method in which one pixel is divided into a plurality of sub-pixels, emission or non-emission is selected in each of the sub-pixels, and the gradation is represented according to a difference between a light emitting area and the other area in a single pixel. The time gradation method is a method in which gradation representation is performed by controlling the emission period of a light emitting element. To be more specific, one frame period is divided into a plurality of subframe periods having mutually different lengths, emission or non-emission of a light emitting element is selected in each period, and the gradation is presented according to a difference in length of light emission time in one frame period. In the digital gradation method, the method in which a digital gradation method is combined with a time gradation method (hereinafter, referred to as time gradation method) is proposed. (For example, refer to Patent Document 1).